Growing crystals can be difficult or easy, depending on what material
it is you are trying to grow a crystal out of. Most pure substances
will make crystals when frozen slowly enough, and many of these can be
made to form crystals if they come out of solution, as in the sugar
example you give.
In fact, it is nearly impossible to prevent crystal formation in
substances which naturally form them. One way to prevent crystal
formation is to freeze the substance very rapidly. How rapidly that is
depends a lot on the type of material. Glass isn't crystalline because
it is cooled off faster than it takes to form regular patterns for the
silicon dioxide molecules. Cool silicon dioxide very much more slowly,
and you might get nice quartz crystals. Pressure may play a role in
forming rock crystals as well, but the usual ingredient is time and
patience, and a pure sample of the material.
Many materials we don't think of as crystalline show their
crystalline structure if you look at them under a microscope. Metals,
sugar, sand, and salt, all have tiny little crystals in them. The big
art form in growing crystals is growing big, perfect ones.
It helps to get the crystallization process off to a good start
with a "seed" crystal around which the crystal will grow. Otherwise,
once the freezing point is reached, many tiny little crystals will
start forming in different places in your material, and they will grow
and collide with each other. If you can start the crystallization in
just one place first, you have a better chance at growing just one big
I'd imagine you can grow salt crystals in the same way you grow
sugar crystals. Before trying to grow crystals of other substances,
please check to see if they are poisonous or flammable, and make sure
that they are solids at room temperature (don't try to crystallize
gasoline, for example -- it's a complicated mixture, it's a liquid at
room temperature, it's poisonous, and it's flammable!).
(republished on 07/13/06)